The general structure of microbolometer detectors is well known in the art. Briefly, they include a two-dimensional array of transducers, each of which is electrically connected to an underlying readout integrated circuit (ROIC) chip that contains circuitry for detecting changes in the resistance of the transducer. The array is often referred to as a “focal plane array” and the individual detectors are referred to as “pixels”. The transducers are formed of a material, such as vanadium oxide, which has a high thermal coefficient of resistance (TCR). To isolate the transducers thermally, they are typically held by a structure that supports them in a suspended position over the ROIC chip. When radiation such as infrared radiation is incident on the transducers in the focal plane array, an image of the source of the radiation is generated.
Several criteria are important in determining the efficiency of a microbolometer detector. First, to increase the responsiveness and sensitivity of the individual transducers, it is important that the support structure isolate them thermally from the ROIC chip. Second, the support structure and transducer must occupy the same pixel area. Therefore, as the size of the pixels becomes smaller (e.g., to 25-50 μm) the support structure tends to take up more space of the pixel area, and a lower portion of the overall pixel area is occupied by the transducers themselves. The percentage of the pixel area that is occupied by the transducers is sometimes referred to as the “fill factor”. To maintain a high quality image it is desirable to keep the fill factor as high as possible, preferably close to 100%.
U.S. Pat. No. 6,144,030 proposes one solution to these problems. Each transducer is supported by “leg members” that are located entirely beneath the transducer, and the individual leg members are formed in a “serpentine configuration” that increases the effective length of the leg members and hence the thermal isolation of the transducers. The fabrication of these serpentine structures, however, may require relatively dense photolithographic processing. This processing density may result in low yields. In addition, the effective length of the leg members, even when they are formed in a serpentine configuration, is limited by the area underneath the transducer.